Ratchet driving tools are well known, both in the form of ratchet wrenches and ratchet screwdrivers. Ratchet screwdrivers are distinct from ratchet wrenches by being of a different shape and importantly, by usually having a lighter construction. The purpose of either tool is similar in that oscillating rotation of a handle causes a drive shaft to progressively rotate. The rotation turns a screw or nut fastener.
Ratchet tools typically function by a toothed pawl engaging splines. The splines are spaced around the either the outside or the inside of a cylindrical surface. In the case of the former the cylindrical surface is part of the drive shaft. In the case of the latter the cylindrical surface is part of the tool body.
An improvement to ratchet tools is a stepless mechanism wherein a wedging action replaces the teeth. Two basic types of stepless actions are known. One design uses discrete rolling elements and transmits torque entirely by containing an efficient expanding force. A second version is a cam wedge brake which combines a wedging action with friction to transmit torque. Since the desire in a driving tool is to transmit torque rather than to contain an expanding force, a device that combines wedging with friction will be most effective.
A sprag clutch falls into the category of rolling elements. For example, U.S. Pat. No. 4,162,000 shows a typical sprag clutch. Eccentric shaped sprag elements are held between an inner and an outer circular race. This type of design is quite common in machinery. A variation of the sprag clutch is shown in U.S. Pat. No. 5,482,144. In this design the sprag elements are wedged in two ways; first between the inner and outer races, and second within angled channels of each race.
Another rolling element clutch is shown in U.S. Pat. No. 4,884,478. In this screwdriver design three round rollers fit between a cylindrical outer wall and an eccentric rotatable shaft. This differs from the sprag clutch in that the shaft is eccentric rather than the rolling elements. Designs similar to the '478 patent are the most common type mechanism for stepless ratchet hand tools. Either three or four rollers are typical.
Another similar design is contained within a wrench handle. U.S. Pat. No. 4,669,339 is distinct from the '478 patent above in certain details. Balls 56 of the '339 patent provide the direction selecting bias in the same manner shown by ball 48 in FIG. 6 of the '478 patent.
Similar roller type clutches have been known for more than 100 years. U.S. Pat. No. 592,213, from 1897, shows a roller clutch with direction selecting means similar to the above '478 and '339 patents.
To transmit high torques a rolling element clutch must have large numbers of rollers as in a sprag clutch used for machinery, or it must be large in size. A simpler clutch using fewer rollers still requires precision tolerances to achieve limited torque.
Various one way clutches of the friction style are known. For instance, U.S. Pat. No. 2,735,324 shows a stepless ratchet wrench using a wedging brake action. A brake element is forced into a gap between the rotating shaft and housing body. Empirical testing has indicated this design is not effective. U.S. Pat. No. 2,766,648 discloses an improved wedging method. The rotating shaft features a "V" shaped outer circumference. A corresponding "V" contoured wedge element presses the shaft in direct proportion to a torque on the handle. The "V" shape functions similarly to an automotive "V" belt, wherein the contact friction is amplified by wedging in the "V". In this case a double wedging action occurs, first from the leverage from the handle, and second from the wedging within the "V".
U.S. Pat. Nos. 3,865,215 and 3,877,556 are quite similar to each other. A rotatable shaft includes an eccentric cam which forces shoe elements outward to press the inside wall of a housing. Both the '215 and '556 patents show embodiments where the shoe to housing interface comprise "V" contoured surfaces. Specifically surfaces 16" in FIG. 6 of the '556 patent, and elements B and P in FIG. 4 of the '215 patent are "V" contoured engagements.
Although some of the above friction drives may be effective in torque, they all suffer limitations of complexity and direction selecting ability. None of the friction clutches disclose a means to change direction. Although this is acceptable for machinery, hand tools require a convenient way to change direction. A need therefore exists for an efficient low cost stepless ratchet tool mechanism with a simple means to switch direction.